Obesity is a prevalent comorbid condition in osteoarthritis (OA)patients that is associated with increased pain and disability. Recent research suggests that two psychosocial treatments may be beneficial for patients with OA: lifestyle behavioral weight management and pain coping skins training. Yet, these two interventions have evolved independently, and have never been directly compared. Further, the combination of these two interventions may be more effective than either alone. Research needs to clarify the processes by which these interventions operate (mediators) and patient characteristics that predict success with each intervention(moderators). This study examines two potential mediators (weight toss, pain coping) and a potential moderator (body mass index [BMI]) of treatment outcomes. Overweight (BMI equal to or more than 30) adults with OA (N =280) will be randomly assigned to 1 of 4 treatment groups : 1) lifestyle behavioral weight management alone, 2) pain coping skills training alone, 3) lifestyle behavioral weight: management plus pain coping skills training, or 4) standard care control. Patients in the lifestyle behavioral weight management alone condition will receive a comprehensive, lifestyle modification program designed to decrease weight through changes in lifestyle, exercise, attitudes, relationships, and nutrition. Patients in the pain coping skills training alone condition will receive training in pain coping skills to improve their ability to control and decrease pain, Patients in the lifestyle behavioral weight management plus pain coping skills training condition will receive both a weight management program and pain coping skills training. Measures of pain, physical disability, psychological disability, joint stiffness, activity, gait, and markers of systemic inflammation and joint tissue metabolism will be collected pre- and post-treatment and at 6- and 12, month follow-up. If the combined intervention is effective, future research could explore ways to make this approach even more cost-effective (e.g. home-based or internet-based treatment.) The information gathered about mechanisms could lead to even more effective interventions that more directly target specific change processes, Future research can also test whether these interventions can be applied to obese patients with other rheumatic and chronic pain problems whose excess weight contributes to increased pain and disability (e.g. obese patients with rheumatoid arthritis, fibromyalgia, and low back pain.)